Isotropic coke has a thermal expansion approximately equal along the three major crystalline axes. This thermal expansion is normally expressed as CTE (i.e., coefficient of thermal expansion) over a given temperature range such as 30.degree.-530.degree. C. or 30.degree.-100.degree. C. Isotropic coke is also indicated by a CTE ratio, which is 10 the ratio of radial CTE divided by axial CTE measured on a graphitized extruded rod. Acceptable isotropic coke has a CTE ratio of less than about 1.5 or a CTE ratio in the range of about 1.0-1.5.
Isotropic coke is used to produce hexagonal graphite logs which serve as moderators in high temperature gas-cooled nuclear reactors. This type of coke has been produced in the past from natural products such as gilsonite. The production of such graphite logs from gilsonite and the use thereof are described in U.S. patents such as Sturges U.S. Pat. No. 3,231,521 and Martin et al. U.S. Pat. No(s). 3,245,880; and 3,321,375 and to Peterson et al. U.S. Pat. No. 3,112,181 describe the production of isotropic coke using petroleum distillates. Contaminants such as boron, vanadium and sulfur have prohibited the use of some materials as the source of isotropic coke suitable for use in nuclear reactors. Less than about 1.6 weight percent sulfur is preferred to avoid puffing problems upon graphitization and fabrication of the coke. The levels of contaminating elements (sulfur, nitrogen, metals) in many cracked petroleum stocks and sweet resids are sufficiently low to produce a relatively pure carbon product when subjected to delayed coking and calcining, but the products from these raw materials are usually too anisotropic. Sour resids produce cokes which are more isotropic, but they usually are too high in contaminating elements.
One method of producing an acceptable product is to airblow a relatively sweet vacuum resid prior to coking it. This method is described in U.S. Pat. No. 3,960,704 in which a residuum, such as bottoms from the fractionation of virgin feedstocks, is air-blown to increase its softening point. The air-blown resid is then subjected to delayed coking to produce isotropic coke having a CTE ratio less than 1.5.
It would be desirable to provide a process in which petroleum stocks containing low levels of contaminants, which have not been subjected to air-blowing, could be converted to isotropic coke.